1. Field of the Invention
The present invention relates to a semiconductor memory device.
2. Background Art
Recently, as a semiconductor memory device expected to replace a dynamic random access memory (DRAM), a floating body cell (FBC) memory device is known. The FBC memory device is formed by NMOS transistors each of which includes a floating body (also referred to as “body region”) on a silicon on insulator (SOI) substrate and each of which can store data “1” or “0” according to the number of holes accumulated in this body region.
An FBC memory consisting of a field effect transistor (FET) has a problem of the following charge pumping phenomenon. In a case that the memory cell is an N-type FET, a part of electrons in an inversion layer are trapped by an interface state on an interface between a gate oxide film and the body region when the memory cell is turned on. Holes accumulated in the body region are recombined with the electrons and disappear. Normally, if a word line is activated between a certain refresh operation and the next refresh operation, no data is written back to unselected FBC memory cells. If the unselected FBC memory cells are repeatedly turned on and off when data is read from and written to the selected memory, therefore, the holes accumulated in body regions of the unselected memory cells each storing data “1” gradually decrease. As a result, data “1” of the unselected memory cells are changed to data “0”, respectively.
Therefore, an FBC memory cell of the FBC memory is not a destructive read-out cell differently from a memory cell of the DRAM but cannot be said to be a complete non-destructive read-out cell. It is discovered that the FBC memory cell is a so-called quasi non-destructive read-out cell.